1. Field of the Invention
The present invention generally relates to a navigation apparatus for moving objects, for instance, automobiles. More specifically, the present invention is directed to a vehicle-mount type navigation system in which both a GPS (Global Positioning System) type navigator and a self-contained type navigator are compatibly operated to accurately detect a present position of the vehicle even when GPS navigation waves transmitted from the satellites are not available in the position where the vehicle is being located.
2. Description of the Related Art
An attention is paid to the fact that a GPS positioning system is very useful in the case where navigation waves are transmitted from a plurality of satellites to various kinds of moving objects such as ships, airplanes, automobiles, and the present positions and moving velocities thereof are confirmed or determined. According to the GPS positioning system, the GPS navigation waves transmitted from a plurality of satellites are substantially simultaneously received, and then the present position, i.e., the two-dimensional or three-dimensional position of a moving object on and above the earth can be recognized.
As is known in the field, the positioning operation which is performed by using such a GPS positioning system is generally executed by receiving the GPS navigation waves transmitted from three or four or more satellites. When the GPS navigation waves from three satellites can be simultaneously received, the two-dimensional position (latitude and longitude) can be determined. On the other hand, when the navigation waves from four or more satellites can be simultaneously received, the three-dimensional position (latitude, longitude, and altitude) can be determined. The GPS navigation waves from a plurality of satellites are simultaneously received by a moving object. A desired correcting process is performed to the time shifts based on the differences of accuracies among timer devices provided for the plurality of satellites and a timer device provided for the moving object. Thereafter, the present position of the moving object is calculated and displayed on a display unit. At this time, the map information relating to the present position is superimposed with the information regarding the present position and thereafter displayed on the display unit.
Various types have been proposed as such a sort of conventional GPS positioning systems; for instance, "The Global Positioning System", by Martin R. Stiglitz, MICROWAVE JOURNAL, April 1986, pages 34 to 59; and "GPS NAVIGATION SYSTEM", Japan Radio Company's Technical Journal, No. 24, pages 16 to 23, issued on Jan. 25, 1986. An arrangement of the latter navigation system is shown in FIG. 1 and will be briefly explained hereinbelow.
In FIG. 1, the GPS navigation waves from a satellite 10 are received by a GPS antenna 11 attached to a moving object (not shown). An output terminal of the antenna 11 is connected to a GPS receiver 12. Pseudo distance data obtained by the GPS receiver 12 is supplied to a GPS positioning calculator 13. The calculator 13 calculates the accurate latitude, longitude, and altitude on the basis of the pseudo distance data, so that the accurate present position of this moving object is detected. The data regarding the present position obtained in this manner is stored into a data memory 15 and displayed as a visual image by a display 14 so as to overlap the related map.
To perform the foregoing two-dimensional or three-dimensional GPS positioning, the GPS navigation waves transmitted from at least three or four satellites among a plurality of satellites which constitute the GPS positioning system, need to be substantially simultaneously received by the GPS navigation system of a moving object. However, there are for example, obstacles such as skyscrapers or high mountains in cities or region sandwiched mountains so that the navigation waves transmitted from a desired number of (three or four or more) satellites cannot substantially simultaneously reach an antenna of a moving object and a desired GPS positioning cannot be performed. On the other hand, even if the GPS positioning may be performed, there is another problem such that the positioning accuracy remarkably deteriorates because of the attenuation of the GPS navigation waves caused by the reflection thereof due to the practical arrangement of the satellites with respect to a moving object or due to obstacles mentioned above.
On the other hand, a self-contained type navigation apparatus has conventionally widely been used as a navigator for various kinds of moving objects as mentioned above.
Different from the foregoing GPS navigation apparatus, in the self-contained type navigation apparatus, the present position of a moving object itself can be known on the basis of only the data obtained from the self-contained type navigator itself without utilizing the navigation data from the outside such as GPS navigation waves from satellites.
On the other hand, a vehicle-mount type navigator having a GPS positioning type navigating function and a self-contained type navigating function has been recently developed, in which either one of these functions is properly, intentionally selected by an operator.
In such a kind of vehicle-mount type navigator, when the GPS navigator function cannot be selected due to the presence of various kinds of obstacles, the self-contained type navigator function is selected and the present position or moving direction of a vehicle itself can be recognized or determined on the basis of only the data which is acquired from a drive-distance sensor or azimuth sensor.
It is now assumed that the GPS navigator function of the vehicle-mount type navigation apparatus is selected. In this case, a vehicle moves on a moving route while receiving the GPS navigation waves from the satellites through the GPS antenna. However, for example, assuming that a high building exists as an obstacle, the GPS navigation waves transmitted from a plurality of satellites cannot be received. Otherwise, the noises like the GPS navigation waves reflected by other disturbances are necessarily received, so that a positioning error is included in the display image of the present position of the vehicle (i.e., multipath noise). That is, on the displayed map, it looks as if the vehicle was remarkably deviated from the moving route. Such a phenomenon is called a "jump phenomenon". There is a fear such that the car driver of the vehicle erroneously confirms the present position due to the jump phenomenon and misdrives the vehicle, so that a serious traffic accident may occur.
The present invention has been made to solve such various conventional problems. It is the first object of the invention to provide a GPS navigation system in which even when the GPS positioning cannot be performed or the positioning accuracy deteriorates, the present position of a moving object an be accurately detected and determined by utilizing the position information of a self-contained type navigator.
A second object of the invention is to provide a vehicle-mount type navigation apparatus having both of the GPS type navigator function and the self-contained type navigator function, wherein in the case where the GPS navigator function is selected, at a time point when the GPS navigation data exceeds a reference value range, the collected GPS navigation data is ignored and simultaneously the navigation data derived by the self-contained type navigator function is used.